The present invention relates to a glass tube processing method and apparatus for performing chemical vapor deposition or diameter shrinkage of a substrate glass tube by relatively moving the substrate glass tube and a heating source in a longitudinal direction of the substrate glass tube, and further a glass tube in which chemical vapor deposition or diameter shrinkage is performed.
In a step of manufacturing an optical fiber preform, an chemical vapor deposition step (see, for example, Non-patent Reference 1) of forming a glass layer inside a glass tube or a step of shrinking a glass tube to a desired diameter is performed. In these steps, a glass tube is sequentially heated in a longitudinal direction of the glass tube by a heating source provided outside the glass tube.
For example, in an chemical vapor deposition step called an chemical vapor deposition CVD method, glass raw material gas for generating glass fine particles (SiO2) is introduced into the inside of a substrate glass tube used as a substrate of chemical vapor deposition and a heating source provided outside the substrate glass tube is transversely moved along a longitudinal direction of the substrate glass tube and the substrate glass tube is heated. By heating the substrate glass tube thus, the glass raw material gas put into the inside of the substrate glass tube is oxidized and the glass fine particles are generated. Then, the glass fine particles are deposited on an inner surface of the substrate glass tube in the downstream side of a stream of the glass raw material gas. Thereafter, the deposited glass fine particles are heated by traverses of the heating source and become transparent and a glass layer is sequentially formed.
Such a chemical vapor deposition step is repeatedly performed and until a wall thickness of the substrate glass tube reaches a desired thickness, a plurality of glass layers are formed and a glass tube forming an intermediate of the optical fiber preform can be formed.
Further, in a diameter shrinkage step, as a previous step of implementing collapse (wherein a glass pipe of which a hollow portion is filled in and becomes a glass rod) of a substrate glass tube by, for example, a collapse method or a rod-in collapse method, the substrate glass tube is heated along a longitudinal direction of the substrate glass tube and is softened and a diameter of the substrate glass tube is shrunk by surface tension similar to that of the collapse method.
[Non-Patent Reference 1]
“Optical Fiber Communications International Edition 1991”, McGraw-Hill Book Co., p. 66-67
By the way, as a heating source used in such glass processing, an oxyhydrogen burner is generally used. When the oxyhydrogen burner is used, since its flame rises upwardly, normally, a substrate glass tube arranged in a horizontal direction is heated by applying the flame from the lower side thereof with being rotated about the axis thereof. In that case, the flame is not applied directly to the upper side of the substrate glass tube, so that it is difficult to obtain uniform temperature distribution over a circumferential direction of the substrate glass tube and circumferential bias occurs in viscosity of the substrate glass tube.
As a result of that, there were cases that a shape of the glass tube is deformed after the processing. Also, there were cases where the softened substrate glass tube is shrinked locally due to wind pressure caused by the flame.
When the glass tube is deformed by the processing thus and its sectional shape becomes oval, trouble about the optical fiber preform is caused.
For example, when collapse (wherein a glass pipe of which a hollow portion is filled in and becomes a glass rod) of a glass tube formed in an chemical vapor deposition step is implemented by a collapse method to form a glass rod and a core portion of the optical fiber preform is formed, a core of an optical fiber obtained from its preform also becomes oval. Then, transmission performances are degraded due to, for example, occurrence of polarization mode dispersion.
Incidentally, such ovality of the glass tube was enhanced remarkably when a diameter of the substrate glass tube is large and a thickness of the substrate glass tube is thin.